This invention relates generally to load responsive fluid control valves and to fluid power systems incorporating such valves, which systems are supplied by a single fixed or variable displacement pump. Such control valves are equipped with an automatic load responsive control and can be used in a multiple load system in which a plurality of loads is individually controlled under positive and negative load conditions by separate control valves.
In more particular aspects, this invention relates to direction and flow control valves capable of controlling simultaneously a number of loads, under both positive and negative load conditions.
In still more particular aspects, this invention relates to automatic synchronizing controls for synchronization of the compensating action of positive and negative load compensators, in controlling fluid flow in and out of fluid motors of a cylinder piston rod type.
In still more particular aspects, this invention relates to negative load compensating control of a compensated direction control valve, in which the negative load throttling action is responsive to the fluid motor inlet pressure, generated by the pump.
Closed center load responsive fluid control valves, of a fully compensated type, are very desirable for a number of reasons. They permit load control with reduced power loss and therefore, increased system efficiency and when controlling one load at a time provide the feature of flow control, irrespective of variation in the magnitude of the load. Such valves are provided with positive and negative load compensating controls, which automatically maintain a constant pressure differential and therefore constant flow characteristics, through the metering control orifices handling the flow in and out of the fluid motor. Such a fluid control valve is shown in FIG. 3 of my U.S. Pat. No. 3,744,517 issued July 10, 1973. However, such fully compensated control valves suffer from one basic disadvantage, when controlling fluid flow to and from an actuator, in the form of a cylinder, which, due to the well-known piston rod effect, is characterized by different flow rates between the in and out flows of the cylinder. Depending on the direction of actuation such cylinders, when controlled by the valve of U.S. Pat. No. 3,744,517, can be subjected either to cavitation, or excessive pressures, due to the energy derived from the pump circuit during control of negative load.
This drawback can be overcome in part by the provisions of the fully compensated proportional valves disclosed in my U.S. Pat. No. 4,222,409, issued Sept. 16, 1980. In this compensated control valve, during negative load control, the pump circuit is automatically isolated from the cylinder, preventing generation of excessive pressures, while the cavitation condition is prevented by fluid flow from pressurized exhaust manifold. This type of control, although very effective, suffers from one serious disadvantage in applications requiring high control stiffness and high frequency response. Those harmful characteristics result from the fact that the energy derived from the pump cannot be directly applied to both ends of the actuator, without going through the stage of isolating the actuator from the pump, during control of negative load. Therefore, such valves display some undesirable characteristics, when used as proportional, or servo valves, in servo systems controlling loads.